1. Field of the Invention
Embodiments of the present invention relate to an optical recording and/or reproducing method and apparatus, and more particularly, to methods and apparatuses compensating for tilt and an optical recording and/or reproducing apparatus implementing the same.
2. Description of the Related Art
A typical optical pickup assembly includes an optical pickup system having a light source, an objective lens, a photodetector, and an actuator for driving the objective lens.
An optical recording and/or reproducing apparatus records information and/or reproduces information on and/or from an optical information storage medium by moving an optical pickup assembly. The optical pickup assembly can be mounted on a shaft of a disc loading device installed in a deck (hereinafter called a “deck shaft”) in a radial direction of the optical information storage medium.
To properly record and/or reproduce information, the amount of tilt of an objective lens, relative to an optical information storage medium, should be within a predefined tolerance range. Accordingly, skew adjustment is performed by adjusting the position of the deck shaft, in radial and tangential directions, on the deck when assembling an optical recording and/or reproducing apparatus. This mechanical skew adjustment process is commonly called a second deck skew adjustment. Similarly, skew adjustment made during the assembly of the optical pickup assembly is commonly called a first skew adjustment.
Meanwhile, a tilt servo control is performed to compensate for a tilt of the objective lens relative to the optical information storage medium. Tilt servo control can be achieved by generating a tilt error signal. However, since it is difficult to generate a tilt error signal under the current technological conditions, when the optical pickup assembly is not optimized for tilt compensation, a DC tilt servo control can be performed to compensate for any initial tilt, e.g., in the case of a DVD.
When reproducing a prerecorded optical information storage medium, such as a DVD-ROM, a ROM signal can be read from the optical information storage medium, a vale of jitter best or RF envelope can thereby be obtained at about 4 to 5 points on the medium while moving the optical pickup assembly from an inner circumference of the medium toward an outer circumference. The amount of DC compensation can then be pre-calculated based on this value. Here, the pre-calculated amount of DC compensation corresponds to a focus DC offset (FODC).
Based on data obtained by evaluating the pre-calculated amount of DC compensation and a best skew point (unique to each optical pickup) of an optical pickup using an evaluator, a tilt of the deck shaft can be compensated for in radial and tangential directions.
However, this approach cannot provide accurate compensation because the FODC may vary when the deck shaft moves inward or outward in a radial direction. A distance by which an optical pickup actuator is driven in a focus direction may also increase.
The problem becomes more severe with a recording apparatus. That is, for a prerecorded DVD disc, a jitter best or an RF envelope can be detected when reading recorded pits, and current can then be applied to an actuator for tilt compensation. Jitter best corresponds to when a measured jitter is within a predefined tolerance range.
However, since it is not possible to obtain the jitter best or RF envelope for an unrecorded DVD disc, as shown in FIG. 1, a FODC value is measured at several points on the disc when moving from the inner to outer circumferences of the disc in order to measure and compensate for an initial tilt amount before recording. A tilt angle is obtained using the measured FODC value and used to compensate for the initial tilt amount. In this way, in the case of the unrecorded DVD disc, a tilt servo control is performed using the FODC value.
According to the conventional approach, mechanical misadjustment may occur when a second deck skew adjustment is performed in a direction opposite to the direction of the tilt resulting from the measured FODC value, as shown in FIG. 1.
Since the second deck skew adjustment is initially made only between the deck and a test disc available from each manufacturer, it leads to a second deck skew in an unrecorded disc mounted on an optical recording and/or reproducing apparatus in actual use, increasing the amount of a tilt.
In FIG. 1, L′ and L″ represent the original position of the deck shaft and the position of the deck shaft subjected to the second deck skew adjustment, respectively. D′ and c′ denote a disc and a rotation axis of the disc D′, respectively. p1′, p2′, p3′ and p4′ denote locations of the disc D′ where measurements of a FODC value are made.
When mechanical misadjustment occurs due to the second deck skew adjustment, the conventional tilt compensation method cannot provide accurate compensation because the distance by which an actuator is driven in the focus direction increases. Furthermore, when the actuator is highly nonlinear in the focus direction, it is difficult to seek an accurate value of FODC due to the possibility of mechanical misadjustment.